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Re: [linux-lvm] Snapshots and disk re-use



On 2/23/2011 1:39 PM, Stuart D. Gathman wrote:
On Wed, 23 Feb 2011, Les Mikesell wrote:

Actually, if you never (or rarely) write to the origin, lots of snapshots
should be fine.

But every write to the origin will first copy the
original origin data to every snapshot.

Why would origin data be copied over to the snapshot after the snapshot
has been created? Surely the point of a snapshot is to have "frozen" data?

Yes, is the way this actually works explained somewhere?  I would have
expected the 'copy-on-write' blocks to be copied only on the side where the
write is happening and relocated instead of rewriting all the snapshots that
might be outstanding with the old data.

It should be obvious when you think about it.

Well, maybe if you've never used a NetApp. They seem to make a big deal out of only writing once.

You go to write to the origin,
to a block you've never written to since creating the snapshot.  Where
are you going to save that original data?  Why in the *-cow LV of course.

The origin and snapshot LVs do not have data of their own.  The
origin data is in *-real, and new writes to the origin first read *-real,
write *-cow, then finally do the new write to *-real.  This read-write-write
cycle is tolerable for a single snapshot, but quickly gets out of hand
with multiple snapshots.

Which is why re-writing stuff that was already correct and not being modified doesn't make much sense.

Writes to the snapshot, on the other hand, simply write to the *-cow with
some mapping, no need to update the origin.

I would have guessed that snapshots were read-only. If you can write, it isn't really a snapshot, is it?

--
  Les Mikesell
    lesmikesell gmail com


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